Amplitude modulator for microwaves



March 31, 1959 2,880,399

E. J. MURPHY AMPLITUDE MODULATOR FOR MICROWAVES Filed Oct. 20, 1952 IIHHII .l I O INVENTOR [aw/m0 L/. MURPHY ATTORNEY Rand Corporation, a corporation of Delaware I Application October 20, 1952, Serial No. 315,727

15 Claims. Cl. 333-41 This invention relates to microwave transmisison systems, and, more particularly, is concerned with low frequency modulation of microwave signals. The general object of this invention is to divide energy from a signal microwave source into two linearly polarized waves that are modulated in time quadrature with respect to each other.

'Another object of this invention is the provision of a modulator in which the total of the modulated energies transmitted to'two separate loads is substantially constant and equal to the total energy output of the microwave source.

,AI Oher Object of this invention is to provide a modulator which produces a minimum of reflection over a fairly wide microwave frequency band.

' Another object of this invention is the provision of antenna means for launching energy in the dominant mode of a circular wave guide from energy transmitted in the dominant mode of a coaxial line.

Another object of this invention is to provide such antenna means which may be mechanically rotated to impart rotation to the plane of polarization of energy launched in the circular wave guide.

These and other objects of the invention which will become apparent as the description proceeds are achieved by providing a section of coaxial line into which microwave energy is coupled and transmitted in the dominant mode. One end of the coaxial line is joined to a circular wave guide section in axial alignment therewith. The

inner conductor of the coaxial line terminates in a novel hook-shaped antenna which extends into the circular wave guide and launches plane polarized waves in the TE mode. A pair of rectangular wave guides are joined to the circular wave guide with their longitudinal axes perfpendicular to each other and to the longitudinal axis of the circular wave guide. Output apertures in the wall of the circular wave guide couple energy into the respective rectangular wave guides. coaxial line is rotated by suitable means at the modulation frequency, rotation of the antenna resulting in rotation of the plane of polarization of the signal launched in the circular wave guide. Rotation of the plane of polarization in turn produces a balanced-modulator type modulation of each of the output signals.

For a better understanding the invention reference should be had to the accompanying drawings, wherein: Fig. 1 is a longitudinal sectional view of a modulator incorporating the features of the present invention; Fig. 2 is a plan view of the modulator of Fig. 1; Fig. ,3 is a cross-sectional view taken on the vIIIIII of Fig. 1; Fig. 4 is a cross-sectional view taken on the line IV-IV of Fig. 1; and

Fig. 5 is an enlarged view of the hook-shaped antenna. Referring to the form of the invention as illustrated in the drawing, the numeral indicates generally a waveline The inner conductor of the guide-to-coaxial-line transition of a type commonly re- 'ferred to as a door knob transition, and includesa Patented Mar. 31, 1959 rectangular wave guide input section 12 to which is perpendicularly joined a coaxial line section 14. One end of the outer conductor 16 of the coaxial line section 14 is connected to a broad wall 18 of the rectangular wave guide section 12. The inner conductor 20 of the coaxial line section 14 in turn is electrically joined to the opposite broad wall 22 of the rectangular wave guide section 12 by means of a knob 24 of proper size and shape to provide impedance matching of the portion of the center conductor 20 within the rectangular wave guide section 12. The power handling capacity of the transition is improved by streamlining the knob 24 in a wine glass shape, as shown. Broad band operation of the transition is further improved by utilizing an inductive iris formed by inserts 26 and 27 in the wave guide section 12. A coupling flange 28 is provided on the input end of the rectangular wave guide section 12 while the other end is terminated in a short-circuiting plate 30. Such transitions are well-known and full described in vol. 9, Radiation Laboratory Series, McGraw-Hill publishers, pages 349-353.

' The outer conductor 16 of the coaxial line section 14 is provided with a coupling flange 32 to which is secured in sealed relationship by means of screws 34, the end flange portion 38 of a circular wave guide 36.

An important feature of the present invention is the provision of a special hook-shaped antenna, indicated generally at 40, which is contiguous with the inner conductor 20 and extends into the hollow region of the circular-wave guide 36. The antenna 40 is designed to launch energy in the circular wave guide in the dominant or TE mode. The shape of the antenna as shown was derived empirically to provide a substantial impedance match between the coaxial line and the wave guide.

The antenna includes a double reverse bend portion 41 followed 'by a substantially straight portion 42 inclined at an angle to the axis of the wave guide 36, and is terminated in a ball portion 44 for improving the broad band performance of the antenna. The antenna has a smaller diameter than the inner conductor 20, thus forming a step 43 in the inner conductor. The shunt capacity'eflect of the step improves the impedance match between the two transmission line sections.

A typical set of dimensions for such antenna matching between the standard coaxial line section having a A" diameter inner conductor and a 1 /2" circular wave guide section is as follows:

Diameter D=%" Length L=1 /s" Radius r =.437

Radius r =.213

Center distance l =.421 Center distance :972 Center distance l =.089 Angle A: Diameter d=%" V The antenna is situated so as to extend approximately 1%" into the circular wave guide 36, the edge of the outer conductor 16 being flared to conform to the curvature of the antenna.

To derive output signals from the circular wave guide 36, a pair of rectangular wave guide output sections are provided, indicated at 46 and 48. The longitudinal axes of the output wave guide sections are mutually perpendicular, and perpendicular to the longitudinal axis of the circular wave guide 36.

The output section 46 is joined to the wall of the circular wave guide 36 to form a sealed joint therebetween, and a rectangular aperture 50 in the wall of the circular wave guide 36 within the area common to the output section 46 provides coupling of energy therebetwen.

Similarly, output wave guide section 48 is secured to the wall of the circular wave guide 36, and an aperture 52 in the wall in the region common to the output section 48 couples energy from the circular wave guide to the output section 48. The output wave guide sections are provided with coupling flanges 53 and 55 respectively.

To elfect amplitude modulation of the energy output at the output sections 46 and 48, the antenna 40 is rotated at the modulation frequency. To provide rotation of the antenna 40, the inner conductor 20. of the coaxial line section 14 passes out through an opening 54 in the knob 24 and has secured to the end thereof'opposite from the antenna 40 a gear 56. The inner conductor 20 is supported for rotation by means of a hollow shaft 58 journalled in a housing 66 by suitable bearing means such as ball bearings 62. The inner conductor 20 is removably secured within the hollow shaft 58 by means of a set screw 64 which may be reached for tightening by removing an access screw 66 in the housing 60. An oil seal, indicated at 67, is seated in a counterbored portion of the knob 24 to provide a rotary seal between the interior of the transmission line and the housing 60.

The inner conductor 20 is rotated by means of a motor 68 which drives, through a chain of speed reducing gears 70, the gear 56 on the end of the inner conductor 20. The housing 64] is secured with screws 74 to a flange portion 72 integral with the knob 24. The entire modulator unit can be supported by a flange portion 76 on the housing 60.

A signal input to the input wave guide section 12 is transmitted by the transition 10 through the coaxial line section 14 and launched in the TE mode within the circular wave wave guide 36 by the antenna 40. The plane of polarization of the waves within the circular wave guide 36 is determined by the orientation of the antenna 40, the E-vector being aligned with a plane defined by the longitudinal axis of the circular wave guide 36 and the intersecting longitudinal axis of the straight portion 42 of the antenna 40. Fig. 4 shows the electric field within the circular wave guide 36 as launched by the antenna 40 in the position shown in Figs. 1 and 3. As the inner conductor 20 is rotated, the plane of polarization is likewise rotated.

Since energy may be coupled through the apertures 50 and 52 only from the electric field component extending across the respective apertures, it will be appreciated that when the electric field is orientated as shown in Fig. 4, no energy is coupled out through the aperture 50 and the maximum amount of energy is. coupled through the aperture 52. When the inner conductor 20 is rotated 90 from the position shown in the drawing, the reverse condition prevails, with the maximum energy being coupled through the aperture 50, and no energy being coupled through aperture 52. The amount of energy coupled through the apertures 50 and 52 thus varies sinusoidally with rotation of the plane of polarization of the electric field within the circular wave guide 36. The energy output in each of the output sections 46 and 48 is modulated at the frequency of rotation of the inner conductor 20, the action of the rotating antenna in combination with the output apertures being equivalent to a balanced modulator in which the output signal is a double-sideband suppressed-carrier type signal. Because the output wave guides are mutually perpendicular, the energy peaks in the respective output signals are phased in time quadrature.

To. reduce the amount of direct coupling between the two output wave guide sections, a septum 78 is placed in the circular wave guide 36 in the region substantially between the two output apertures Stl and 52. Theseptum 78, by being positioned close to the aperture 51 also improves the match at the junction between the circular wave guide 36 and the output wave guide section 46 and thereby improves the broad band operation of the modulator. Similarly, a septum 8,0 is positioned in the, cir- 42 cular wave guide near the aperture 52 to improve matching between the circular wave guide 36 and the output wave guide section 48. The circular wave guide section is terminated in a short-circuiting plate 82.

From the above description, it will be recognized that the various objects have been achieved by providing a modulator in which simultaneous modulation of the two output signals is effected by a single rotating element. By proper matching and careful design, the sum of the combined energies coupled to the two output wave guide sections is always constant, so that the load seen by the microwave source is likewise constant. The particular hook-shaped antenna utilized provides an efiicient mode converter between the coaxial line section and circular wave guide and readily lends itself to mechanical rotation for modulation purposes. The modulator as described may be used for high power applications, and since no resonant elements are involved, is operable over a substantial frequency hand. For example, when used over a frequency band of 5400 to 5900 megacycles, the VSWR for the entire modulator unit is less than 1.3.

While certain portions of the modulator have been referred to in the specification and claims as input and output portions, it is to be understood that the energy may be transmitted in either direction through the modulator. Use of the words input and output is therefore not to be construed in a limiting sense.

Since many changes could be made in the aboveconstruction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A microwave modulator comprising a circular hollow pipe wave guide section, a coaxial transmission line section connected to the circular wave guide section and in axial alignment therewith, a rectangular hollow pipe input wave guide section, the outer conductor of the coaxial line section being connected at the end thereof remote from the circular wave guide section to a broad wall of the input wave guide section and the inner conductor of the coaxial line section extending transversely through the input wave guide section, a hook-shaped antenna contiguous with the inner conductor of the coaxial line section and extending into the circular wave guide section, means for rotatably supporting the inner conductor of the coaxial line section, means associated with the inner conductor for imparting rotation thereto, and at least one rectangular output wave guide section joined to the outer surface of the circular wave guide section, the axes of propagation of the circular wave guide section and output wave guide section being perpendicular to each other and the wall of the circular wave guide section having a rectangular aperture therethrough adjacent the output wave guide section for coupling energy from the circular wave guide section to the output wave guide section.

2. A microwave modulator comprising a circular hollow pipe wave guide section, a coaxial transmission line section connected to the circular wave guide section and in axial alignment therewith, a rectangular hollow pipe input wave guide section, the outer conductor of the coaxial line section being connected at the end thereof remote from the circular wave guide section to a broad wall of the input wave guide section and the inner conductor of the coaxial line section extending transversely through the input wave guide section, a hook-shaped antenna contiguous with the inner conductor of the coaxial line section and extending into the circular wave section, means for rotatably supporting the inner conductor of the coaxial line section, means associated with the inner conductor for imparting rotation thereto, and at least one rectangular output wave guide section, cou- 3. A microwave molulator comprising a circular hollow pipe wave guide section, a coaxial transmission line section connected to the circular wave guide section and in axial alignment therewith, a hook-shaped antenna contiguous with the inner conductor of the coaxial line section and extending into the circular Wave guide section, means for rotatably supporting the inner conductor of the coaxial line section, means associated with the inner conductor for imparting rotation thereto, and at least one rectangular output wave guide section coupled to the circular wave guide section at right angles thereto.

4. A microwave modulator comprising a circular hollow pipe wave guide section, a coaxial transmission line section connected to the circular wave guide section and in axial alignment therewith, a hook-shaped antenna contiguous with the inner conductor of the coaxial line section and extending into the circular wave guide section, means for rotatably supporting the inner conductor,

of the coaxial line section, means associated with the inner conductor for imparting rotation thereto, and means for coupling an energy component from the circular wave guide section polarized in a particular plane.

5. Apparatus comprising a circular hollow pipe wave guide section, a coaxial transmission line section connected to the circular wave guide section and in axial alignment therewith, a hook-shaped antenna contiguous with the inner conductor of the coaxial line section and extending into the circular wave guide section the hookshaped antenna launching plane polarized energy in the dominant TE mode in the circular wave guide, and means for rotating said antenna about the central longitudinal axis of the circular wave guide section for rotating the plane of polarization of said energy launched in the circular wave guide by the antenna.

6. In combination, a section of coaxial transmission line, a section of circular wave guide joined thereto in axial alignment with the coaxial line section, and antenna means having a circular cross-section and being contiguous with the inner conductor of the coaxial line section and projecting into the circular wave guide section, said antenna means including a double reverse bend portion adjacent the inner conductor, a straight line portion within the circular wave guide section inclined at an angle to the longitudinal axis of the circular wave guide, and a ball portion at the end of the straight portion remote from the inner conductor of the coaxial line section, the antenna means having a smaller diameter than the inner conductor of the coaxial line section.

7. In combination, a section of coaxial transmission line, a section of circular Wave guide joined thereto in axial alignment with the coaxial line section, and antenna means contiguous with the inner conductor of the coaxial line section and projecting into the circular wave guide section, said antenna means including a double reverse bend portion adjacent the inner conductor, a straight line portion within the circular wave guide section inclined at an angle to the longitudinal axis of the circular wave guide, and a ball portion at the end of the straight portion remote from the inner conductor of the coaxial line section.

8. In combination, a section of coaxial transmission line, a section of circular wave guide joined thereto in axial alignment with the coaxial line section, and antenna means having a circular cross-section and being contiguous with the inner conductor of the coaxial line section and projecting into the circular wave guide section, said antenna means including a double reverse bend portion adjacent the inner conductor, and a ball portion at the end of the antenna means remote from the inner conductor of the coaxial line section, the antenna means having a smaller diameter than the inner conductor of the coaxial line section.

9. In combination, a section of coaxial transmission line, a section of circular wave guidejoined thereto in axial alignment with the coaxial line'section, and antenna means contiguous with the inner conductor of the coaxial linesection and projecting into the circular wave guide section, said antenna means including a double reverse bend portion adjacent the inner conductor, and a ball portion at the end of the antenna means remote from the inner conductor of the coaxial line section.

10. In combination, a section of coaxial transmission line, a section of circular wave guide joined thereto in axial alignment with the coaxial line section, and antenna means having a circular cross-section and being contiguous with the inner conductor of the coaxial line section and projecting into the circular wave guide section, said antenna means including a double reverse bend portion adjacent the inner conductor, and a straight line portion within the circular wave guide section inclined at an angle to the longitudinal axis of the circular wave guide, the antenna means having a smaller diameter than the inner conductor of the coaxial line section.

11. In combination, a section of coaxial transmission line, a section of circular Wave guide joined thereto in axial alignment with the coaxial line section, and antenna means contiguous with the inner conductor of the coaxial line section and projecting into the circular wave guide section, said antenna means including a double reverse bend portion adjacent the inner conductor, and a straight line portion within the circular wave guide section inclined at an angle to the longitudinal axis of the circular wave guide.

12. In combination, a section of coaxial transmission line, a section of circular wave guide joined thereto in axial alignment with the coaxial line section, and antenna means having a circular cross-section and being contiguous with the inner conductor of the coaxial line section and projecting into the circular wave guide section, said antenna means including a double reverse bend portion adjacent the inner conductor, the antenna means having a smaller diameter than the inner conductor of the coaxial line section.

13. In combination, a section of coaxial transmission line, a section of circular wave guide joined thereto in axial alignment with the coaxial line section, and antenna means contiguous with the inner conductor of the coaxial line section and projecting into the circular wave guide section, said antenna means including a double reverse bend portion adjacent the inner conductor.

14. Apparatus comprising a section of coaxial transmission line, a section of circular wave guide joined thereto in axial alignment, antenna means contiguous with the inner conductor of the coaxial line section and projecting into the circular wave guide section, said means including a double reverse bend portion, with the end of the antenna means remote from the inner conductor of the coaxial line section terminating at a point substantially off the central longitudinal axis of the circular wave guide section, whereby said antenna couples energy from the coaxial line section to the circular wave guide section, launching the energy in the circular wave guide section in the plane polarized dominant TE mode and means for rotating said antenna means within the circular wave guide section for rotating the plane of polarization of energy coupled into the circular wave guide section.

15. Apparatus comprising a section of coaxial transmission line, a section of circular wave guide joined thereto in axial alignment, antenna means contiguous with the inner conductor of the coaxial line section and projecting into the circular wave guide section, said antenna means including a double reverse bend portion, whereby said antenna couples energy from the coaxial line section to the circular wave guide section, launching the energy in the plane polarized dominant TE mode, and means for rotating said antenna means within the circular wave guide section for rotating the plane of polarization of energy coupled into the circular wave guide section.

References Cited" in the file of this patent UNITED STATES PATENTS 8 Lindenblad Oct. 26, 1948 Feldman K L Jan. 11,1949 Cady June20, 1950 Hansen 2 July 1-1, 1950 Clapp t May 20, 1952 Ramsay et a1. Dec. 16, 1952 Zaleski Feb; 10, 1953 

